Observations by surface and airborne Sun photometers and satellite sensors have noted significant changes in aerosol properties in proximity to clouds. These studies have noted increases in aerosol optical thickness of 5-25% in “transition zones” of a few kilometers to several tens of kilometers away from clouds. These increases may be due to a several factors such as: hygroscopic growth of the aerosol particles, increased particle production near clouds, in-cloud processing of aerosol particles, and in the case of satellite measurements, cloud adjacency effects and cloud contamination of satellite pixels.

Data from two lidar systems are used to examine the behavior of aerosols in these transition zones. The lidar measurements are unaffected by cloud adjacency effects, are less susceptible to cloud contamination, and possess high vertical and temporal resolution to capture variations in aerosol optical properties near clouds. The ground-based U.S. Department of Energy Atmospheric Radiation Measurement Climate Research Facility Raman lidar provides a detailed view of the variability of aerosols and water vapor near clouds at or near the top of the Planetary Boundary Layer. Aerosol and water vapor properties in the vicinity of clouds at the top of the daytime boundary layer are examined using 10 second profiles of aerosol backscattering, water vapor mixing ratio, and relative humidity, and 1 minute profiles of aerosol extinction in conjunction with continuous Total Sky Imager images of cloud cover.

Data from the NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) are also used to examine the variability of aerosol optical properties near clouds. The LaRC airborne HSRL measures aerosol backscatter and depolarization at 532 and 1064 nm and aerosol extinction at 532 nm. HSRL data acquired over central and northern Oklahoma during the Cumulus Humilis Aerosol Processing Study (CHAPS) field campaign and Cloud and Land Surface Interaction Campaign (CLASIC) conducted by the Department of Energy during June 2007 are used. The campaigns involved several coordinated air- and ground-based remote and in situ sensors, including the airborne HSRL aboard the NASA King Air B-200 aircraft, and instruments aboard the DOE Gulfstream-1 and Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. The aircraft were flown in coordinated patterns over common ground tracks on several occasions.

Preliminary results using these Raman lidar and HSRL data show that aerosol backscatter and extinction decreased by approximately 25-40% moving 1-2 km away from clouds. The variations in aerosol optical thickness are smaller, typically around 10-15%, since the changes in aerosol backscatter and extinction were generally confined to the top of the boundary layer. Variations in the Raman lidar aerosol backscatter and extinction measurements were typically confined to altitudes 200-400 m below cloud base and 200 m above cloud base. Raman lidar observations show relative humidity decreased by 5-15% within this same region suggesting that variations in backscatter and extinction near clouds were caused in part by hygroscopic aerosol growth. Variations in the HSRL aerosol depolarization measurements also suggest that aerosol nonsphericity changed in response to variations in relative humidity.

This presentation will discuss the use of these lidar measurements as well as airborne in situ measurements to study the behavior of aerosols near clouds.